1. Field of the Invention
The invention relates to fluid seals and more particularly to a fitting for a high pressure seal that must be periodically disassembled and replaced with a low pressure seal arrangement. The invention has particular utility in connection with a nuclear reactor wherein the high pressure seal between the thimble guide tube and the thimble at the seal table on the exterior of the well in which the reactor vessel is seated must be released and replaced with a low pressure seal during the refueling of the reactor.
2. Description of the Prior Art
Refueling of pressurized water reactors is an established, routine operation carried out with a high degree of reliabilty. Refueling is normally performed once or twice a year depending upon load requirements. For the sake of economy, it is desirable that the refueling operation be accomplished as quickly as possible. In recent years a number of design innovations have considerably simplified refueling operations, reducing the number of operator actions required during refueling and, hence, considerably reducing the amount of time for a complete refueling operation from approximately four weeks, to less than seven days. The present invention is generally directed toward further simplifying fying the refueling operations, minimizing the downtime of the reactor during such refueling, and increasing the reliability of certain components which must be manipulated during refueling.
In a typical pressurized water nuclear reactor arrangement, the reactor vessel is seated in a concrete well and contains the usual nuclear reactor core and instrumentation. Among the instrumentation are a plurality of stainless steel tubes, referred to as thimbles, which extend, during normal reactor operation, from the upper plate of the core downwardly to the bottom of the vessel where they pass through a penetration in the bottom of the vessel and terminate at a point exterior to the vessel well. The thimble is normally empty, however, approximately once a month a neutron flux detector is pushed through the thimble to the top of the core and then slowly retracted while neutron flux readings are taken with suitable flux mapping equipment to which the detector is connected outside the thimble.
The thimble itself is housed in a larger stainless steel tube, referred to as a guide tube, which stainless steel tube, referred to as a guide tube, which is welded to the bottom of the vessel and which forms the vessel penetration through which the thimble passes. The interior of the guide tube is exposed to the reactor cooling water and hence the operating pressure of the system, whereas the inside of the thimble is essentialy dry and at atmospheric pressure. The guide tube extends on the exterior of the vessel from the vessel pentration through the concrete wall forming the well of the vessel to a stainless steel plate, known as the seal table. A termination stub of the guide tube penetrates the seal table and is welded thereto. The thimble passes entirely through the guide tube and extends beyond the seal table toward the flux mapping equipment.
Because the space between the outer surface of the thimble and the inner surface of the guide tube is exposed to the operating pressure of the reactor, it is necessary during normal reactor operation to have a high pressure seal at the seal table at the point where the thimble exits the guide tube in order to prevent reactor coolant from being expelled from the guide tube.
During a refueling operation, it is necessary to retract the thimble a number of feet in order to remove it from the volume of the nuclear core. To do this, the high pressure seal at the seal table between the guide tube stub and the thimble is disassembled so that the thimble can be retracted the necessary distance. Although the reactor is powered down during the refueling operation and the water pressure within the vessel is concomitantly reduced, it is still necessary to have a low pressure seal between the guide tube stub and the thimble because the seal table is approximately at or below the upper flange of the vessel and is thus below the water level of the refueling canal which is normally filled during the refueling operation.
In the past it was necessary to temporarily lower the water level within the vessel to a point below the seal table during the time that the high pressure seal at the seal table was disassembled and a low pressure seal installed in its place. Thereafter, the vessel and refueling canal would be filled with water to provide sufficient shielding to keep the radiation within an acceptable level when fuel assemblies are removed from the vessel.
The prior technique of physically installing a low pressure seal at the seal table after disassembling the high pressure seal, and then, after the refueling operation, subsequently reinstalling the high pressure seal, has several disadvantages. First of all, it is time consuming because the water level has to be reduced to a level below the seal table each time the seal between the guide tube stub and the thimble is replaced. Further, the action of changing from a high pressure seal to a low pressure seal, and vice versa, is time consuming in that, depending on the design of the reactor, there could be in excess of 50 thimbles, each of which must be retracted through a separate guide tube stub at the seal table. Another disadvantage is that the reliability of the seal at the seal table between the guide tube stub and thimble could be impaired in the course of changing from one to the other type of seal. The reason for this is that the high pressure seal is a compression type seal employing a compression ring, or ferrule, which is compressed via a lock nut to form a friction-tight, fluid seal at the exterior of the guide tube stub. When the high pressure seal is disassembled, the ferrule remains in place and is utilized as an anchor point for the compression nut in the formation of the low pressure seal. If during this conversion from a high pressure seal to a low pressure seal, the ferrule slips, the integrity of the low pressure seal may be impaired. Further, and more importantly, when the high pressure seal is reassembled the ferrule, once having slipped, would have to be removed and a portion of the guide tube stub may have to be cut off in order to provide a clean, unmarred surface for the compression fitting of the high pressure seal. Since the guide tube stub only projects beyond the seal table by several inches, the entire guide tube may eventually have to be replaced in order to provide a sufficiently long guide tube stub on which to form a high pressure compression seal.